Combustion engines such as gas turbine engines are machines that convert chemical energy stored in fuel into mechanical energy useful for generating electricity, producing thrust, or otherwise doing work. These engines typically include several cooperative sections that contribute in some way to this energy conversion process. In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section or combustion chamber. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and to achieve desired performance characteristics. One popular combustor design includes a centralized pilot burner (hereinafter referred to as a pilot burner or simply pilot) and several main fuel/air mixing apparatuses, generally referred to in the art as injector nozzles, arranged circumferentially around the pilot burner. With this design, a central pilot flame zone and a mixing region are formed. During operation, the pilot burner selectively produces a stable flame that is anchored in the pilot flame zone, while the fuel/air mixing apparatuses produce a mixed stream of fuel and air in the above-referenced mixing region. The stream of mixed fuel and air flows out of the mixing region, past the pilot flame zone, and into a main combustion zone of a combustion chamber, where additional combustion occurs. Energy released during combustion is captured by the downstream components to produce electricity or otherwise do work.
It is known that high frequency pressure oscillations may be generated from the coupling between heat release from the combustion process and the acoustics of the combustion chamber. If these pressure oscillations, which are sometimes referred to as combustion dynamics, or as high frequency dynamics, reach a certain amplitude they may cause nearby structures to vibrate and ultimately break. A particularly undesired situation is when a combustion-generated acoustic wave has a frequency at or near the natural frequency of a component of the gas turbine engine. Such adverse synchronicity may result in sympathetic vibration and ultimate breakage or other failure of such component.
Various resonator boxes for the combustion section of a gas turbine engine have been developed to damp such undesired acoustics and reduce the risk of the above-noted problems. FIG. 1A provides a perspective view of a prior art combustor liner 10 with a resonator section 11. As shown, along a cylindrical region 20 of the combustor liner 10 are respective arrays 12 of apertures 13 of adjacent resonators. Resonators 14 are shown complete with resonator boxes 15 in place, and two arrays 12 of apertures 13 are shown with the resonator boxes 15 removed.
As shown in FIG. 2 the resonator box 15 has sidewalk 16 that are welded to an outside surface of the combustion liner 10. In addition, the resonator boxes 15 have an array of impingement air holes 17 on a top plate or wall 18, an array of impingement holes 17 typically follows the same geometric shape of the array 12 of apertures 13 on the liner 10 and the air holes 17 are typically staggered relative to the apertures. In addition, thermal barrier coatings are disposed on the interior (exposed to combustion gases) surface of liner 10 respectively upstream and downstream of the cylindrical region 20 of the liner 10 which comprises the resonators 14, but not throughout the cylindrical region 20, which remains uncoated. The uncoated region is predominantly cooled by a combination of cooling from the impingement air holes 17 and film cooling from air flow exiting through the apertures 13.
As the demands on power generation increase and turbines are designed for more efficient production of power output, the operating temperatures of the turbine components increase. In particular, the temperatures within combustion chambers are ever increasing as a result of higher firing temperatures, use of alternative fuels or fuel flow biasing from different injection stages, for example. To provide sufficient cooling, making the apertures in the liner larger will increase NOx emissions and will not provide protection against oxidation. Accordingly, a thermal barrier coating (TBC) is needed along the inner surface of the combustion liner at the region defined by the resonators, at a resonator section of a combustion liner.
However, current masking techniques to cover apertures during deposition of a thermal barrier coating are too time consuming and costly. In addition, typical masking materials such as polymer masking cannot be used with some deposition techniques such as dense vertical cracked TBC, which may destroy the masking material during deposition. In addition, some masking techniques, such as the use of polymer materials to clog and cover holes, and as done in the fabrication of other components of a turbine engine, form an uncontrolled halo or undercoating around the apertures because of manual processes involved. If halos exist at the apertures, then the resonator would not meet its targeted frequency requirements. In addition, weld heat generated when welding the boxes directly to the outer surface of the liner may damage the TBC. Moreover, whether or not a TBC is applied to a resonator region of a liner, the weld at the surface of the liner creates a high stress area as the combustion chamber operates at such high temperatures.